EARLY MOLECULAR SPECIFICATION IN THE HIPPOCAMPAL RUDIMENT - ISOLATIONOF GENES EXPRESSED IN A REGION-SPECIFIC MANNER IN THE EMBYRONIC TELENCEPHALON

During development, the structure of the telencephalon is dramatically transformed by region-specific proliferation and differentiation of n euroepithelial cells. To understand mechanisms of morphogenesis in the telencephalon, a search was made for genes expressed in a region-spec ific manner in the dorsal telencephalon of embryonic rats. Dorso-later al and dorso-medial portions of the telencephalon were taken from rat fetuses on embryonic day (E) 13. Genes expressed in each portion were compared by a modified differential display method. Among more than 40 00 cDNA fragments, four fragments termed M1, M1', M2 and M3 were found only after amplification of cDNAs derived from the dorso-medial porti on of the telencephalon. Nucleotide sequence and Northern blot analyse s showed that M1 and M1' were identical and that M2 and M3 were relate d genes. In situ hybridization revealed that M1 was expressed in a res tricted portion of the hippocampal rudiment with a sharp boundary. M2 and M3 were expressed in the hippocampal rudiment with a more diffuse boundary than that of M1. Expression of M2 and M3 was also found in th e most anterior part of the medial diencephalon with a caudal extensio n along the dorsal midline. These results suggest that at least two ty pes of molecular specification occur in the dorso-medial telencephalon , contributing to the construction of the hippocampal elements during early stages of development.